This invention relates to the treatment of blood coagulation pathologies. The invention generally deals with novel methods for preparing new compositions useful in the management of hemorrhagic episodes in patients with inhibitors of blood clotting factors. In particular, the invention is concerned with the therapy of antihemophilic factor or plasma thromboplastin component inhibitors.* FNT *The term inhibitor in this patent refers to a specific antibody to either antihemophilic factor in Hemophilia A patients or to plasma thromboplastin component in Hemophilia B patients.
Blood coagulation is an exceedingly complex process. The interaction of various blood components which eventually gives rise to a fibrin clot has been compared to a cascade of steps, each of which is dependent upon and regulated by preceding and following steps. Generally, the blood components which take part in the coagulation cascade are either proenzymes or enzyme modulators. The proenzymes are enzymatically inactive proteins which are converted to proteolytic enzymes by the action of an "activator", generally another proteolytic enzyme produced at an earlier stage in the coagulation cascade. Coagulation factors which have undergone such a conversion are hereafter defined as activated factors, and designated by the lower case postscript "a" while the proenzymes are referred to as precursor clotting factors.
The enzyme modulators are principally cofactors such as calcium ions or nonenzyme proteins and most are essential if the enzymes are to exhibit any catalytic activity at all. Such modulators are to be distinguished from enzyme substrates. Substrates are compounds which are covalently modified by an enzyme while modulators or cofactors merely bind to the enzyme without undergoing a change in structure.
Blood coagulation is best visualized as a cascade of reactions between formed and soluble blood components in which most segments of the cascade are demarked by a proenzyme. An initial event such as the contact activation of Hageman factor (factor XII) will start one branch of the cascade. Basically, the product of this initial event then activates the next proenzyme in the cascade and so on in a sequential process until a fibrin clot is formed, the total rate of the cascade being dependent upon cofactors and modulators.
While much is now known about the various blood coagulation factors and the manner in which they interact and are susceptible to environmental influences, there remain many areas where questions remain. One such area is the action, etiology and therapeutic treatment of blood clotting factor inhibitors.
Inhibitors of blood clotting factors pose substantial difficulties in conventional hemorrhage therapy. Uncontrolled bleeding due to coagulation deficiencies is usually halted by supplying the deficient component from pooled plasma sources. However, since inhibitors are frequently elucidated by the patient in response to the presence of the deficient clotting factor, this conventional approach is frequently counterproductive since increasing the dosage of clotting factor merely results in greater output of inhibitor. This problem with clotting factor inhibitors is not an isolated one. For example, up to an estimated 21% of the Hemophilia A population develops factor VIII inhibitor, i.e. antihemophilic factor (AHF) inhibitor. To fully understand the background of this invention it is necessary to discuss the mechanisms of blood clotting and the influence of inhibitors thereon.
The etiology of such clotting factor inhibitors is not well defined, but is thought to be along two principal routes. First, as explored above, frequent infusions of therapeutic clotting factor concentrates such as AHF frequently produce an immune response in patients as evidenced by increased inhibitor titer. Multiple challenges of the patient's immune system with AHF are believed to stimulate ever-increasing AHF antibody levels. Since such antibodies may then complex with the AHF and block its activity, the increases in antibody titer dictate greater doses to achieve a satisfactory clinical response.
In contrast, the second route of appearance for the inhibitors is not believed to be a function of the administration of therapeutic blood protein fractions. Rather, the inhibitor seemingly arises spontaneously in the manner of an idiopathic or autoimmune disease, frequently following on the heels of drug reactions or collagen disorders.
The medical community has dealt with clotting factor inhibitors by (a) administering either extremely low or extremely high doses of the clotting factor which is being inhibited, with or without immunosuppression, (b) using clotting factor of non-human origin or (c) administering activated prothrombin complex concentrates (PCC), i.e., PCC in which at least a small proportion of the clotting factors have been converted to active enzymes. The first two techniques have not been widely used. The infusion of sufficiently large amounts of clotting factor to overwhelm the inhibitor existing in the patient's system become less and less effective with each treatment episode because inhibitor titers rise in response to each administration of clotting factor. On the other hand using non-human clotting factor creates a risk of severe immune reactions in treated patients.
The use of activated PCC for the treatment of patients afflicted with clotting factor inhibitors has received widespread acceptance, following a presentation by Fekete et al. at the XIV International Congress of Hematology in 1972. For example, see Kurczynski et al., "New England Journal of Medicine" 291(4): 164 (1974) wherein an activated PCC containing 15 units of factor II/ml (the term "factor" will be frequently abbreviated herein as "F"), 200 units of F-VII/ml, 42 units of F-IX/ml, 58 units of F-X/ml, 3-10 units of F-IXa/ml, 3-8 units, of F-Xa/ml and 0.001-0.003 units of thrombin/ml was used therapeutically to treat F-VIII inhibitor afflicted patients. The clinical success of such concentrates has been ascribed to the presence of the various activated clotting factors VIIa, IXa or Xa, or thrombin, although the identity of the operative activated factor or factors is subject to controversy. Recently, success has also been attributed to the presence of a component possessing "factor eight inhibitor bypassing activity" (FEIBA). This component is not believed to be one of the activated factors, II, VII, IX or X, but otherwise the nature of its activity is not well defined.
Activated PCC, when used to treat factor-VIII inhibitor, has the advantage that the complex can be tailored to be sufficiently free of factor VIII antigen that an immune response in humans are not observed.
The activated factors which are present in most of the prothrombin complex concentrates previously used to treat clotting factor inhibitors are artifacts of the plasma fractionation procedure in which prothrombin complex is enriched from Cohn fraction I supernatant; the activated factors were not induced by any special steps and as a result were often considered to be in too low or too variable a concentration to be satisfactory.
While processes for the activation of PCC have been generally alluded to in the art, the only detailed disclosure known to applicants of a protocol for manufacturing such products appears in U.S. Pat. No. 4,160,025, to Eibl et al. These patentees urge that before their method was developed, activated prothrombin complex concentrates "could not be tested as regards their effective principle and could not be standardized . . . therefore the results [were] not safe and [could] hardly be repeated." The patentees go on to state that their method "has as its object to safeguard in a repeatable and deliberate manner a generation of the desired factor-VIII-inhibitor-bypassing activity" (two paragraphs bridging columns 1 and 2).
The Eibl et al. method comprises activating a starting material selected from plasma, cryoprecipitate-poor plasma or Cohn fraction I supernatant by use of a contact activator, followed by absorption of the FEIBA component and factors II, VII, IX and X onto a basic ion exchanger. Contact activators are well known substances such as silica or kaolin which initiate the intrinsic coagulation mechanism by activation of Hageman factor.
While Eibl et al. are highly concerned with standardizing their final product they give scant attention to the activation procedure. The pH, temperature, starting materials and activators are generally described but no mention is made of the activation period other than the one or three hours disclosed in Examples 1 and 2.
It is extremely difficult to avoid excessive activation of prothrombin complex concentrates because the activation reactions, being an enzyme cascade, tend to accelerate rapidly at variable and largely unpredictable rates which are controlled by substances in the activated sample and by the kinetics of the enzymes in question. The most potentially harmful result of excessive activation is the appearance of thrombin, or activated factor II in the product. For example, Eibl et al. report thrombin levels of 0.05 and 0.07 NIH units/ml. Thrombin is not considered desirable because it is capable of acting directly on blood components to yield a fibrin clot while other activated clotting factors exert their effect earlier in the coagulation cascade and hence are more likely to be subject to modulation by blood components in vivo. The elevated thrombin levels reported by Eibl et al. are believed by applicants to be a function of the failure of Eibl et al. to adequately control the activation procedure. Eibl et al. do not screen the starting material for activation, thus failing to take into account the pre-existing activation state of each lot of plasma or plasma fraction used, and do not determine the in-process response of the lot to activation. However, Eibl et al. do suggest in Example 1 that variations in the clotting factor and FEIBA levels in various lots of final product may be compensated for by mixing bulk batches until the desired ratio of clotting factors to FEIBA is achieved. This is unsatisfactory because of costs, yield losses and contamination risk inherent in such a procedure. Further, thrombin is frequently undesirably elevated, even in products which were apparently manufactured by following this procedure.
According to White et al, "Blood" 49 (2): 159-170 (1977), American Red Cross PCC is nonthrombogenic in part because of the presence of heparin and the deliberate fortification with antithrombin III. These two additives are said to result in the irreversible inactivation of proteases in PCC. White et al. did not report treating inhibitor patients with such PCC; it is problematic that such a concentrate would be useful for this purpose where any activated factors which might inadvertently be present, as well as mechanisms for their generation, are suppressed by heparin and antithrombin III.
It is accordingly an object of this invention to standardize activated PCC preparations without mixing or handling the finished product.
It is an additional object to control the manufacture of activated PCC to reduce the production of thrombin during the process and to neutralize any thrombin which is produced.
It is another object to treat patients having clotting factor inhibitors or deficiencies with an activated PCC containing selected activities of factors II, VII, total IX, X, VIIa, IX precursor, and Xa.
These and other objects of the invention will be apparent to those skilled in the art from consideration of the specification taken as a whole.
The principal object of this invention is accomplished by determining in advance of the completion of activation the conditions needed to achieve an activated PCC of substantially predetermined composition. This is in contrast to the passive approach to the unsolved problems of standardization and thrombogenicity which characterizes the published prior art, where conditions such as the time and temperature of activation are arbitrarily set and any difficulty with the resulting product is remedied, if possible, by selecting lots which when combined will yield the desired products. Accordingly, in a method wherein a prothrombin complex-containing blood protein fraction is activated under conditions which produce enzymatically active blood clotting factors, the improvement comprises
(a) selecting at least one of said conditions which is to be varied to control the degree of activation;
(b) prior to the completion of activation, determining the magnitude of the condition needed to activate the fraction to a predetermined degree of activation;
(c) setting the condition to said magnitude; and
(d) conducting the activation of the fraction in accordance with said condition.
Generally only one condition of the activation is permitted to vary, and this is usually the period of time that activation is allowed to proceed.
The magnitude of the selected condition is determined in one of two ways, or a combination of both. In the least preferred of the two methods, the condition is determined by removing aliquots of the fraction after activation has been commenced, terminating the activation of each aliquot, determining the degree of activation of each aliquot and calculating the magnitude of the condition necessary to achieve a predetermined degree of activation of the fraction.
Alternatively, the condition magnitude may be determined by removing aliquots of the fraction prior to activation, varying the condition among the aliquots, activating the aliquots in accordance with the condition set for each aliquot, terminating the activation, determining the degree of activation of each aliquot and calculating the magnitude of the condition necessary to achieve a predetermined degree of activation of the fraction. This embodiment has the advantage that one cannot overrun the predetermined activation level, as could be done during the assay of aliquots withdrawn from a bulk lot which is simultaneously undergoing activation.
Control of the activation process is also facilitated by selecting as starting materials only fractions which exhibit a low degree of spontaneous activation.
The degree of activation is generally monitored by following the nonactivated partial thromboplastin (NAPT) or factor VIII correctional times although thrombin determinations are also useful. These assays are fully described below. The levels of individual clotting factors, may also be determined as a measure of activation. Methods for determining these factors are also described herein.
An additional object of this invention is accomplished by activating an intermediate PCC produced during the method of U.S. Pat. No. 3,560,475. Selection of a particular point in the patented method to activate the PCC greatly facilitates the control of the activation procedure because of the presence in the PCC of an activation retardant.
A further object is achieved by adding heparin to the activated product after the activation retardant has been neutralized or removed, generally immediately before the product is filled into containers and lyophilized.
A further object is accomplished by including the stabilizers heparin and, optionally, antithrombin III in the final activated PCC. These substances inactivate thrombin and are believed to provide a margin of safety against thrombosis in susceptible patients, e.g., those with liver dysfunction.
This invention also includes an improved activated PCC composition which comprises an aqueous solution having clotting factor activities, in units/ml, of F-II, 1-10; thrombin, less than about 0.003; F-VII, about from 37 to 190; F-VIIa, about from 8 to 80; total F-IX, about from 15 to 112; F-IX precursor, 0 to about 30; F-X, about from 1 to 30; and F-Xa, about from 1 to 10. More particularly, the improved activated PCC will contain certain delineated levels of total F-IX and F-IX precursor, F-VII and F-VIIa, F-X and F-Xa, F-VIII correctional activity and NAPT time, and will be sufficiently free of factor VIII antigen to not produce an immune response in patients to whom the activated PCC is administered.